Is it possible to have seamless material blending on two separate objects?
I was working on expressions with shape keys and kept running into issues where manipulating the mesh of a body would break the face and vice versa. I disconnected the two for more control (i.e. two objects), but ran into this issue again. I figured I'd just fix the edge in post, but happened to notice a difference when adjusting the subsurf value.



Looks like the way light bounces around inside of any given object will generate darker spots where objects overlap. Handy to know for certain effects. It also happens to be the reason for my hard edge issues. (educated guesses here)

Here's a quick and dirty demonstration with four separate objects:





The 'hardness' of that clipping edge is dictated by the level of Subsurface applied. Top two renders have the value at .5, and the bottom two are 0:



Fairly simple once I realized what was going wrong. It's worth noting that a combined group of meshes (single object) won't have this problem since the surface is treated as a whole instead of separated bits.